The subject matter disclosed herein relates generally to tuning a motor drive and, more specifically, to a method for providing settings in a motor drive system resulting in improved tuning-less performance for customers.
As is known to those skilled in the art, motor drives are utilized to control operation of a motor. According to one common configuration, a motor drive includes a DC bus having a DC voltage of suitable magnitude from which an AC voltage may be generated and provided to the motor. The DC voltage may be provided as an input to the motor drive or, alternately, the motor drive may include a rectifier section which converts an AC voltage input to the DC voltage present on the DC bus. The motor drive includes power electronic switching devices, such as insulated gate bipolar transistors (IGBTs), thyristors, or silicon controlled rectifiers (SCRs). The power electronic switching device further includes a reverse conduction power electronic device, such as a free-wheeling diode, connected in parallel across the power electronic switching device. The reverse conduction power electronic device is configured to conduct during time intervals in which the power electronic switching device is not conducting. A controller in the motor drive generates switching signals to selectively turn on or off each switching device to generate a desired DC voltage on the DC bus or a desired motor voltage.
The motor drive receives a command signal which indicates the desired operation of the motor. The command signal may be a desired position, speed, or torque at which the motor is to operate. The position, speed, and torque of the motor are controlled by varying the amplitude and frequency of the AC voltage applied to the stator. The motor is connected to the output terminals of the motor drive, and the controller generates the switching signals to rapidly switch the switching devices on and off at a predetermined switching frequency and, thereby, alternately connects or disconnects the DC bus to the output terminals and, in turn, to the motor. By varying the duration during each switching period for which the output terminal of the motor drive is connected to the DC voltage, the magnitude of the output voltage is varied. The motor controller utilizes modulation techniques such as pulse width modulation (PWM) to control the switching and to synthesize waveforms having desired amplitudes and frequencies.
In order to convert the command signal to the desired output voltage, the motor drive includes a control section. The control section may vary in complexity according to the performance requirements of the motor drive. For instance, a motor drive controlling operation of a pump may only need to start and stop the pump responsive to an on/off command. The motor drive may require minimal control such as an acceleration and deceleration time for the pump. In contrast, another motor drive may control a servo motor moving, for example, one axis of a machining center or an industrial robotic arm. The motor drive may need to not only start and stop the motor, but operate at various operating speeds and/or torques or follow a position command. The motor control may include multiple control loops, such as a position, velocity, torque, or current control loop, or a combination thereof. Each control loop may include, for example, a proportional (P), integral (I), or derivative (D) controller and an associated controller gain value for each controller in the control loop and may further require additional feedback and/or feed forward controller gain values. In order to achieve the desired operating performance of the motor, it is necessary to properly select the controllers and the associated controller gain values associated with each control loop.
However, selecting the controllers and associated controller gain values may be a complex process. Adjustment of a controller gain value in one control loop may impact performance of another control loop. Although the control loops may be in parallel or in series with each other, there is ultimately a single input and a single output for the control system. Adjusting a controller gain value along one loop impacts the performance of one or more other controller gain values. The interaction of controller gain values often requires a time and labor-intensive iterative approach to selecting gain values in order to achieve the desired level of performance.
The dynamics of a load connected to the motor typically requires further adjustment of gain values. A load that is rigidly coupled to the motor usually requires different gain values than a load that has a compliant coupling or a coupling with backlash between the load and the motor. Gain values are lowered as the level of compliance and/or backlash increases. As the ratio of load inertia to motor inertia increases, the effects of compliance and backlash are amplified. For example, a coupling with a small level of compliance and a high ratio of load inertia to motor inertia typically results in significantly lower controller gain values than for the same coupling and a low ration of load inertia to motor inertia. Controller gain values that produce desired performance with a rigidly coupled load may excite resonant operating points with a compliantly coupled load. Consequently, varying levels of compliance and/or backlash result in a unique set of controller gain values for each application. Successfully setting the controller gain values to achieve a desired level of performance for each application typically takes time and requires a skilled technician. Many companies don't have such a technician and may need to hire a field service technician from the manufacturer of the motor drive. This can add significantly to the cost to start up and commission a new control system. Some companies may elect to set the controller gain values to a reduced performance level to ensure stability of the controlled system. However, the reduced performance level may result in lost revenue during operation due to operating at less than maximum capacity.
Thus, it would be desirable to provide a motor drive with settings which result in improved tuning-less performance for customers.